RS-232 -- Technical Information

Serial port wiring

This  article details  the connections (pinouts)  of EIA  standard RS-232C.
This connector  consists of a  D-shaped plug with  25 pins in  two rows: 13
pins in the upper row and 12 in the lower.  This interface is commonly used
by devices  that require  a serial interface  to a computer;  these devices
include modems, terminals, serial printers, and such specialized devices as
bar-code scanners.   In addition,  this articles  gives the pinouts  of the
nine-pin DB-9P connector, which is a nine-pin version of the RS-232 that is
commonly used in AT and AT-compatible computers.

RS-232 Pinout

The following table gives the 25-pin EIA standard RS-232C pinouts.  It also
gives:

    ->  Nine-pin DB-9P convention
    ->  Common abbreviations of signal names
    ->  Abbreviations of RS-232 signal names
    ->  Equivalent CCITT signal-number designations
    ->  Signal direction (as appropriate)
    ->  Signal description

Please note that in this  table, DTE stands for ``data terminal equipment''
and refers to  terminal-type equipment such as a PC  or a terminal, whereas
DCE stands  for ``data communications equipment'' and  refers to modems and
modem-type equipment.

DB-25 DB-9 Common
Pin # Pin #Name   EIA CCITTDTE-DCE Description
  1         FG    AA   101   --    Frame ground
  2    3    TD    BA   103   ->    Transmitted data
  3    2    RD    BB   104   <-    Received data
  4    7    RTS   CA   105   ->    Request to send
  5    8    CTS   CB   106   <-    Clear to send
  6    6    DSR   CC   107   <-    Data set ready
  7    5    SG    AB   102   --    Signal ground
  8    1    DCD   CF   109   <-    Data carrier detect
  9         --    --   --    --    Positive DC test voltage
  10        --    --   --    --    Negative DC test voltage
  11        QM    --   --    <-    Equalizer mode
  12        SDCD  SCF  122   <-    Secondary carrier detect
  13        SCTS  SCB  121   <-    Secondary clear to send
  14        STD   SBA  118   ->    Secondary transmitted data
  15        TC    DB   114   <-    Transmitter clock
  16        SRD   SBB  119   <-    Secondary receiver clock
  17        RC    DD   115   ->    Receiver clock
  18        DCR   --   --    <-    Divided clock receiver
  19        SRTS  SCA  120   ->    Secondary request to send
  20   4    DTR   CD   108.2 ->    Data terminal ready
  21        SQ    CG   110   <-    Signal quality
  22   9    RI    CE   125   <-    Ring indicator
  23        --    CH   111   ->    Data rate selector
  24        TC    DA   113   <-    Transmitted clock
  25

Files

/usr/pub/rs232 -- On-line version of above table

See Also

Administering COHERENT,
asy,
modem,
terminal
Seyer, M.D.:  RS-232 Made Easy: Connecting  Computers, Printers, Terminals,
and Modems.  Englewood Cliffs, NJ, Prentice-Hall Inc., 1984.

Notes

Serial ports on  the back of the PC use  either a 25-pin male (DB-25P) or a
nine-pin  male (DB-9P)  connector.  Due  to  what can  only be  regarded as
extreme stupidity, the 25-pin  female (DB-25S) connector was chosen for the
parallel  (printer)  port,  rather than  using  the  usual 36-pin  parallel
connector.  Do not confuse these ports when wiring custom-cable assemblies,
as you can damage your equipment!

© 2012 by Stephen A. Ness